overhead valve- camshaft installed in engine block, valves are operated through lifters, pushrods, and rocker arms. outdated, bad timing at high rpm, simple, durable. difficult more than 2 valves.
OHC or COHC
overhead cam- camshaft installed in the cylinder head, valves operated directly through lifters. valves operated almost directly by camshaft, requires timing bolt or chain, easy perfect timing at high rpm. possible 3-4 valves
DOHC or Twin Cam
double overhead cam- mostly in new cars (w/ 2cams). more complex/expensive. produce more power w/ smaller engine volume. adopt variable timing.
a rotating shaft within the engine that opens and closes the intake and exhaust valve in the engine